caste certificate

Sir,
I have a friend who belongs to scheduled caste and she is married to a person of different caste( obc).
I would like to know whether her son can choose his mother`s caste and get a scheduled caste certificate?
The are residing in Bangalore and the boy is born in Bangalore.
I was told they can get a Scheduled caste certificate from Chennai from where they come from because the rules are favorable to them in Tamilnadu. Is it so?
Is that there are different rules in different states?
Thanks
Sivakumar